Transmission of frequency modulated waves



Oct. 14, 1941. I I H, NYQUIST 2,259,000

TRANSMISSION OF FREQUENCY MODULATED WAVES Filed May 21, 1938 5 Sheets-Sheet 1 FIG.

A TTORNEV Oct. 14, 1941. NYQUIST 2,259,000

TRANSMISSION OF FREQUENCY MODULATED WAVES Filed May 21, 1938 3 Sheets-Sheet 2 N 5 N TOR mvrau/sr Get. 14, 1941.

Fla. 3

AMP & FREQ LIM/TER azuoo H. NYQUIST 2,259,000

TRANSMISSION OF FREQUENCY MODULATED WAVES Filed May 21, 1938 3 Sheets-Sheet 3 Ill 6' EL E C Til/E NETWORK //v l/ENTOR H. NVQU/ST A TTOR/VE Y Patented Oct. 14, 1941 "UNITED STATES PATENT v 7 2,259,000 TRANSMiSSION QF LATED whvns' Harry Nyquist, Millburn', N. o assig trw'nll Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application May 21,

respohaihgiy increasing the interference or noise introduced because of the relatively wide r c ee of frequencies mplo ed.

'Another objector the invention is to reduce the relative efiec-t with respect to the desired signal of interference which consists of very sudden iinpiil ses. 'Iii accordance with the invention, incoming frediiency mo ulated Siren/es are demodulated in any desired'rn'anner after first being subjected to the selective action of a network having the ,geiieral'charac'terifstics of a band-pass filter. In order to excludethe i r'iajoif fiortion of the interferenc'e energy to which the ordinaryfrequency inodiil'aition 'receivihg system is subjected the network i's'given a transmission range onlywide hbfigh'tb readily accommodate essential bands which are required at the moment to give a good tifiality'detectedsigiial. 1h order to perinit the network to continue tortransmit the received waves cals their frequency shifts over a Wide range the network is made Variable and is controlled by the detec'ted currentsto cause its transmissio'h belh'd 'Whil'e remaining narrow to shift up down in the frqu eiiby Scale in step With the f reiiiieiicy of the incoming waves. This may be ecomphshed by causing the'demodulated waves to control the effective reactance or an element of the network such as an inductance Winding a'seo'ciated with an iron core. H I n the drawings, Fig 1 illustrates a high fregfiency station'for reception of frequency modulatd waves; I t

Fig. 2 shows a modification of the circuit of Fig. 1 provided with ineans "for counteracting the distortion occasioned by the variable magnetic inductance; and v v Fig. 3 illustrates anbther modification in Which the automaticall varyin selective networkof Figs. 1 and 2 is empleyed in afrequency moduiated wave detection 'Systein of th 'rwise converition'al type. v I h i v Referring tdFiLg. 1, the incoming antenna or receiving circuit 4 is connected by transformer 5 to ".the input circuitsof two detectors ,each of iivhi'chdmay be of the electron discharge type, Shiintd 'acros's "the 'transfornier inpiit circuit is 1938, S ri aI'NO. 209,183

vs iec we network indicated .gen era l iy w en the ihrorni he'rectehele As illustrated, it may take the gform; of an, anti-resonant circuit tuned'tohave its resonance frequency at approximatey the iininooulated carrierv frequency .of the incoming'carrier wave, VIl'recircuitinay be o dfe gne'd. t an it 11 fier f slnbstantiaiiy. finite impedance to j: ents ofo thetnorrnal modulated carrier frequency but will reduce .by about 3' de'cih' st he voltage ofjconip'ohen'ts which garner rr'cm th'eh'r a1 "ffeiiiiency b their; xiisignal waye byQWhiChtth-bhfiier White is i'nodgi} lated at the remqfetr shutter. .For xample, in the c se ofjhig'h 'quality siiee'chfin whic it siritilegtoffti aiisniitconiponenfifof all s lxtefidiig. "111) vto "3,000 cycles the cirw tune to the norm l lumnodi ilated y'ahd'nlayfintrodnce aioss 0L3 decibels for: "tfoinotiiz'e, '.for'c es whic ,fifepait 3,000 cycleslfrom 'therion'rial 'carrkr frequency.

'hectedfwi'th the 10 cm bi erifiable; ire u nqy rally y t e brok nline ill Ht %S.,; lust ated.

, p ar eee ic llwhicl; w l iw' h" hi'e fil ctr de"type ii ii grid circuits inductively W? ckv-rfil n IQ p o; The grid hi t inc udes A v miir si i ariin ctan eiel n With'th et ma et qcore 5; ancifja.

controlling 'uliiidin'g 1'6, together with a, capacity eliementtl'fl H 'e' I sciliator H. nrodnces oscillations' of s'ubs't" y the natiiral resonanceirequen j the r u t .3- Th t ng o ci cuit is is, T e, ep ndent u'ficn. the magnitude of the indtaii'e e'leinent fih In order to set ch c'rmaii a hitugie that thecpscilq rouhceq "will 'l ce of the normalor ir ir PFIWZ CX YQF the. nqomin H e Bis inclpded 'nge ifisuwithqbiasihg r co roil ng winding lficto determinethe normal" tomo'tive force to which the core l5 isjfsfiihi cted x, The 'seiective network B iss-inrilar totheptuned circuit l3 a a inc ludes a inductance elementiS similar to the'ingncta nce element l4 andwhich is controlled in the same way Joya magnetic core 2B and a control winding}! connected in series th s "is 's'u lat on.

5'5 When n rmal u'ninoduiated time freiident tjafid. a coinmon input Wing;

waves are received over circuit 4, selective networks 6 and I 3 are both tuned to the received wave frequency. Consequently, the full electromotive force of the received Waves is impressed upon the primary winding of transformer 5 and a corresponding electromotive force is applied from the secondary winding of the transformer 5 to the input circuits of detectors I and 8. The oscillator H produces oscillations of the same frequency, namely, normal unmodulated carrier frequency, and applies them to the common portion 9 of the input circuits in phase quadrature with the incoming oscillations transmitted by transformer 5. The result of the combination of the two electromotive forces of this same frequency and in phase quadrature by the detectors is to produce no response in the outputcircuits' of the detectors so that the circuit of source l8 and the series windings l6 and 2| is unaffected. The circuit 6 which, as previously explained, is anti-resonant at the unmodulated carrier frequency waves serves as a by-pass to substantially divert from the transformer 5 other disturbing electromotive forces which may be of substantially more than 3,000 cycles frequency difference from the received unmodulated carrier wave.

Whenever frequency modulation begins at the remote transmitting station the incoming wave at the receiving station will depart from the normal unmodulated carrier frequency by an amount determined by several factors including the modulation index of the transmitting system, the intensity of the modulating signal and the rate at which the intensity of the modulating signal is changing. The tendency will be for the frequency of the band of essential incoming components to shift to such an extent that it will depart from the range which the circuit 6 is normally set to exclude. However, as the frequency of the incoming wave departs from that of the normal unmodulated carrier, the initial condition of phase quadrature at the input circuit of detectors I and 8 between the received wave and the locally produced wave will no longer subsist. The change in frequency of the incoming wave will cause the electromotive force impressed by transformer 5 to fall more nearly in phase with the local oscillation in one detector and to increase the phase difference angle in the input circuit of the other detector.- Consequently, the space current in one detector as, for example, in detector 1 will tend to rise with a consequent increase in the potential difference across the resistance 22 in the output circuit of that detector. In converse manner, the space current of detector 8 will diminish as will the potential difference across the resistance 23 associated with that detector. Whereas points 24 and 25'at the outer extremities of the two resistances were originally at equal potentials so far as the effect of space current is concerned, they now tend to assume different potentials and consequently there is-elfective in the circuit of the source it an additional electromotive force which may either aid or oppose the source I8 according to the polarity of the latter. In accordance with the invention the source I8 is so poled that the effect of the additional electromotive force set up between the points 24 and 25 by the unbalance in the detector circuits is to so change the bias or control current through the winding l6 as to cause the reactance of the element M to change the tuning of the loop I3 in the same direction that the frequency of the incoming waves has changed. In other words, if the instantaneous effect of the frequency modulation is to increase the frequency of the incoming waves, the effect of the unbalance potentials between points 24 and 25 is to correspondingly increase the frequency of the oscillations from the source ll. As will be readily understood, the ideal relationship for balance requires that the phase quadrature between the incoming waves and the locally produced waves be maintained. This condition may be specified if at all times the locally produced oscillations faithfully follow the frequency of the incoming waves with no delay. When the frequency of the incoming waves shifts in the opposite direction from that which has been previously considered, the tendency will be for an unbalanced potential to be set up between the points 2'4 and 25 in the opposite direction thus influencing the reactance Hi to change in such manner that the oscillations produced by the source H keep in step with the frequency of the incoming waves. It will therefore be apparent that the system is one in which there is a tendency to automatically maintain the phase quadrature relation and hence an equality of frequency between the locally produced oscillations and the incoming waves.

Reverse feedback circuits of the stabilized amplifier type are very well known in the electrical transmission art. It may perhaps be appreciated that the circuit of detectors 1 and 8, biasing circuit of source l8 and control winding I6 and oscillator l l is a species of reverse feedback circuit in which the audio frequency detected components are fed back from the outputs of the detectors "i and 8 through frequency modulation of the oscillator l I in such manner as to reduce the resultant detected component. If the amplification which the energy undergoes in the reentrant loop involving the detectors and the feedback circuit is made very high the tendency of the circuits to introduce distortion is reduced and the effective detected audio frequency electromotive force between the points 24 and 25 will tend to faithfully reflect the variations in frequency of the incoming wave. Accordingly, there will be produced between the terminals of the series resistance 26 an electromotive force corresponding in its wave form and frequency to the modulating waves at the remote transmitter. These demodulated signal waves may therefore be transferred through transformer 21 to a telephone receiver 28 or other indicating device.

In the foregoing discussion of the operation of the receiving circuit of Fig. 1, the effect of the unbalance potential between points 24 and 25 in tending to maintain phase quadrature relation between the incoming and the locally produced oscillations has been described. There is an additional effect which the unbalance potential produces through the agency of the selective circuit 6. As the frequency of the incoming wave departs from the normal unmodulated carrier frequency there is a tendency for the selective network 6 to increasingly attenuate the incoming wave because of its frequency discriminatory characteristics. However, the unbalance potential causes the current in control winding 2| to change with a consequent readjustment of the reactance l9 in such direction as to shift the resonance frequency of the resonant circuit 6 in the same manner that the resonance frequency of the resonant circuit I3 is shifted. Accordingly, as the tuning of the circuit l9 changes with the change of frequency of the incoming carrier the desired band of essential frequencies is excluded by the circuit 6 and is effectively transmitted to the transformert while frequency components which were previously transmitted to the transformer are now attenuated. In this manner the selective network 6 follows the incoming wave frequency accepting a relatively narrow band of essential frequencies and suppressing components of frequencies outside that band. As the noise energy is distributed throughout the entire range which the incoming wave traverses only a small portion of it will fall within the passed band at any particular instant; the major portion falling within the suppressed or highly attenuated poi"- tion of the range. Thisexpe'dient therefore enables the receiver to accept the essential frequencies of the desired signal even though these frequencies vary from instant to instant and at the same time to exclude the major portion of extraneouswaves occurring in the over-all range of theincomingsi'gnal.

v Assuming that in the course of modulation the incoming waves and the transmission band of the selective network or vario-filter 6 will traverse a range of frequencies extending 60,000 cycles at eachside of the normal unmodulated carrier frequency and assuming that the eife'ctive range of frequencies unsuppressed by the selective network is 6,000 cycles, the average interference will be reduced so that the improvement amounts to aboutlO decibels. However, the most common form of interference is not usually a steady form but one which consists of very sudden impulses. When a very sudden impulse is passed through a band filter having a 6,000-cycle band-pass range instead of through a band filter of the same gen:

eral type having 60,000-cycles band-pass range 7 not only is the power in the narrower filter onetenth of that in the wider filter but in addition the duration of the disturbance in the narrower filter is ten times as great as in the wider filter. Consequently, the voltage power for the duration of therdisturbance is reduced one hundredfold. For this reason the narrow range filter of applicants invention effects a particularly important result in reduction of noise produced by Very sudden impulses.

Although the filter is very effective in reducing noise it may in some instances introduce certain forms of distortion in the wave which would not otherwise be present. In order to counteract the effects of such distortion, the circuit of Fig. 2 may be employed.- In thiscircuit similar elements are designated as in the circuit of Fig. 1. The circuit of Fig. 2 ,differsfrom that of Fig. 1 in the introduction between the output terminals of the oscillator II and the transformer; 9. ll] of a 'vario-filter 31, made as nearly like the vario-. filter 6 as is practicable and similarly controlled by a control winding 29 associated with the variomagnetic reactance winding 38 serves to nullify the distortion eifects produced by the filter 6 by causing the variable shunting and distorting action of network 3'l upon the local oscillations to correspond at all times to the variable shunting and distortingaction of network 6 upon the received waves so that the phase quadrature condition is maintained.

In Fig. 3 the system for controlling the variofilter 6 and for varying the frequency of the local oscillator H and of its output vario-filter 31 is identical with that shown in Fig. 2. It will be observed therefore that across points and 39 between thevario-filter 6 and the transformer 5 there may-be derived a narrow band of the essential frequencies of the incoming frequency modulated wave which has been largely "ed rrcm the normally attendant extraneous noise pulsesand other interfering waves by the action of the vario filter. I It is, therefore, possible to connect a circuit such as 3| to these points and to associate it with a frequency demodulation receiver of conventional type, as indicated. The de modulating-system in this instance involves the transformer 32,"th e combined amplifier and Hill-'- iter 33 of well-known type, the frequency 'de modulator 34; audio frequency transformer 35 and a signal indicator'iiii. 7 It will be apparent that in this system, as in those of Figs. land 2; the advantage of the automatically controlled var -'filter feature isfully attained.-

What is claimed is: I

i. The method of receiving carrier waves are frequency modulated in accordance hon-periodic signals and of reducing the effect "of disturbing energy of frequencies differing from the instantaneous frequency of the received c r; rier waves which comprises highly attenuating all received components differing in frequency by more than a prassigned value from the stantaneous frequency of the received modulated waves so as tol'eave relatively unattenuated band of substantially fixed width but the limiting frequencies of which constantly change and at any instant are determined by the instantaneous frequency of the received waves and frequency demodulating the unattenuate'd frequency band components whereby signals correspondin'g' to the original modulations maybe derived and noises incident to the attenuated frequency ranges may bee'liminated. I

The method of receiving carrier waves ire-- quehcy modulated by non-periodic waves which comprises accepting only a limited band of fre-' quen'cies essential for demodulation and varying the limits of the accepted band in thesame f'rueasy direction and to the same frequency ex: tent as the instantaneous frequency of there ceived modulated waves varies so as to deriv at all times a band of accepted waves capable upon frequency modulation of yielding non periodic waves corresponding to those "by which the coming modulated waves-are frequency modulated. 3. The method of reception of waves frequency modulated in accordance with non-periodic signals for reducing the effects of disturbing energy of f'requencies differing from the instantaneous frequency of the received carrier waves which comprises suppressing all received frequencies outside of abandof those essential for detection lying between the instantaneous frequency and upper and lower limiting frequencies bothof which continually vary so as to remain 'at rela-' tively fixed frequency separations from the iii-j stantaneous received frequency, detecting the band of un'suppressed waves and causing the de= tected waves to shift the band which is sun' suppressed so that the limiting frequencies of the band effectively follow the instantaneous frequency of the incoming frequency modulated waves. I I p 4. 'Ifhe method 'ofre'ceiving carrier waves which havebeen frequency modulated in accordance with signals comprising selecting from the carrier waves a limited band of frequencies, theband extent {or which relmainssubstantially constant and the limiting frequencies of which follow the frequency'of the incoming waves, the selected wave band local os'cillations'wl'iifi are in frequency agreement and in phase quadrature with the instantaneous frequency of the incoming waves, utilizing energy of any component resulting from lack of frequency agreement and phase quadrature agreement to change the frequency of the local oscillations to restore the phase quadrature relation, and utilizing a portion of the energy of the same component to reproduce the signals by which the received waves are frequency modulated.

5. A system for receiving waves frequency modulated in accordance with non-periodic signals comprising a selective network having an effective transmission band of sufficient width to accept the components of incoming waves of the essential frequencies required for detection, means connected thereto for detecting the waves transmitted by the network, and means under the control of the detected waves for varying characteristics of the network to change the frequency limits of the band which it transmits in the same direction and to the same extent as the instantaneous frequency of the incoming waves varies without changing the width of the band thus causing the central frequency of the transmission band of the network to follow the instantaneous frequency of the incoming waves.

6. A system for receiving waves frequency modulated in accordance with signals comprising a receiving circuit, an attenuating network connected thereto to suppress all components outside a band of preassigned frequency extent centering at the instantaneous received frequency and to select the preassigned band, a frequency demodulating system connected to the receiving circuit to demodulate the selected band of frequencies, and means responsive to the resulting demodulated energy to vary the selective characteristics of the network to cause the central frequency of the selected band characteristic of the attenuating network to follow the instantaneous frequency of the received frequency modulated Waves. '7. In combination, a receiving circuit for frequency modulated carrier waves, a balanced demodulator having input terminals connected thereto and ontput terminals, an indicating circuit connected to the output terminals, a local oscillator having output terminals connected to the demodulator to impress local oscillations thereon for combination with the frequency modulated waves received by the receiving circuit, a tuned circuit having a magnetic core inductance shunted across the receiving circuit, a similar tuned circuit connected to the oscillator to determine the frequency of the oscillations produced, and a control circuit also connected to the output terminals of the demodulator and to the inductances of both tuned circuits to control the magnitudes of the inductance and to cause the tuning of each tuned circuit to follow the instantaneous carrier frequency of the received carrier waves.

8. The method of receiving frequency modulated waves carrying signal modulations while at the same time reducing the effect of undesired disturbances caused by components of frequencies remote from the instantaneous received frequency comprising selectively transmitting a band of the received waves lying between two limiting frequencies which are separated from each other by a substantially fixed amount and both of which vary in frequency in like manner so as at any instant to be separated from the varying received carrier frequency by respective frequency differences which remain substantially constant of suflicient width to include the essential frequencies required for detection and suppressing frequencies outside the limiting frequencies of the band, detecting the selected band of received waves and causing the detected waves to control the limiting frequencies of the band of waves selected in such manner as to cause the central frequency of the selected unsuppressed band to follow the instantaneous frequency of the incoming waves.

9. The method of receiving frequency modulated waves free from the effects of undesired disturbances occurring in the neighborhood of the instantaneous received frequency comprising selection of a band of frequencies of preassigned frequency extent centering at the instantaneous carrier frequency and the limiting frequencies of which vary in consonance with the instantaneous received frequency so as to remain at substantially fixed separations therefrom, demodulating the selected band of frequencies and utilizing the demodulated waves to vary the limits of the selected band in order to cause its central frequency to follow the instantaneous carrier frequency of the incoming waves.

10. The method of receiving carrier waves which are frequency modulated in accordance with signals which comprises selecting from the incoming waves a band of preassigned frequency extent, the limiting frequencies of which are constantly changing so as to maintain the central frequency of the band at substantially the instantaneous carrier frequency of the incoming waves, combining the selected waves with locally produced waves of substantially the instantaneous carrier frequency and in phase quadrature therewith and deriving from the combined waves correcting energy to control the limiting frequencies of the selected band and the phase quadrature agreement between the local oscillations and the instantaneous incoming carrier frequency waves.

11. A system for receiving carrier waves which are frequency modulated in accordance with nonperiodic modulating forces comprising a receiving circuit to which electromotive forces corresponding to the frequency modulated waves may be applied, a frequency selective network capable of transmitting a range of frequencies constituting only a small portion of the frequency range throughout which the frequency modulated carrier wave shifts during its frequency excursions, a wave detector connected to the output terminals of the network to receive from the receiving circuit waves of the limited range of frequencies passed by the selective network and means for causing the pass band of the network to vary in such manner as at all times to accept and transmit to the wave detector the incoming frequency modulated wave.

12. A system for receiving carrier waves which are frequency modulated in accordance with modulating forces comprising a receiving conductor upon which electromotive forces corresponding to the frequency modulated waves may be impressed, a detecting device and a selective network having input terminals connected to the conductor and output terminals connected to the detecting device, the selective network having a transmission band extending over a small fraction only of the range throughout which the frequency modulated wave varies during its frequency excursion and means responsive to waves yielded by the detecting device for varying the frequency position of the transmission band of the network to enable it at all times to accept waves of the instantaneous frequency of the frequency modulated carrier wave.

13. A system for receiving frequency modulated waves comprising an incoming conductor on which energy of the waves may be impressed, a selective pass network for passing a band of a frequency extent which is a small fraction only of the extent of the band through which the frequency modulated Wave ranges during the course of the frequency modulation, a network having input terminals connected to the incoming conductor and output terminals, a receiver for frequency modulated waves connected to the output terminals and means for automatically controlling the frequency position of the band passed to the receiver by the network without substantially varying its width whereby the network at all times accepts the incoming frequency modulated wave irrespective of the character of the modulation involved and of the manner in which the frequency of the incoming wave varies.

14. The method of reception of carrier waves which are frequency modulated in accordance with non-periodic modulating forces which comprises selecting from the received frequency 

